1. Field of the Invention
The present invention relates to a multi-cylinder type injection molding apparatus provided with a plurality of injection cylinders movable in accordance with a mold'prue positions.
2. Descriction of the Related Art
In a conventional injection molding apparatus, each mold is provided with one injection cylinder, except where two-color molding or multi-layer molding is desired.
If the molding has a large size or area like a vehicle bumper or an instrument panel, however, the mold must be provided with a plurality of gates opening into the mold cavity so that resin materials can be injected into all portions of the cavity. The gates are connected to the sprue by runners.
In the mold structure available according to the above method, however, as the molding gets larger, the gate-to-gate size gets longer and the runner section in the mold gets larger. As a result, a larger proportion of the injected resin is held in the runner section, and yield is lowered. A further disadvantage of the enlarged runner section is that the passage for resin from the sprue to a gate gets longer and, thus, molding conditions become more unstable.
To improve the yield rate and molding conditions, thermoplastic resin materials typically are injected through a hot runner in the front cavity of the mold and thermosetting resin materials are injected through a cold runner in the front cavity so that moldings can be obtained without causing resin materials to be hardened at the sprue and runner.
Since the mold equipped with the hot runner must incorporate a great number of parts, however, its structure is complicated, and thus, producing such mold requires much time and cost. Moreover, resin often leaks from seams of the parts comprising the hot runner, and material remaining in the hot runner combines with resin subsequently injected during a series of production runs, resulting in uneven or improper resin mixtures.
To address these problems, multi-cylinder type injection molding equipment with a plurality of injection units has been developed to eliminate the hot runner and improve injection molding conditions (see, for example, the specification of the Japanese Utility Model Publication No. Sho 58-180026).
A prior art injection molding apparatus, as shown in FIG. 1, includes on the base frame 102 of the main body 101 two injection units 108 each having a hopper 117 used to supply resin materials, a fixed head 103 and a movable platen 104 used to mount a mold 105 to be charged with molten resin from an injection cylinder 111 of the injection unit 108, and a clamping cylinder 106 used to open and close the mold 105 via the movable platen 104.
The injection units 108 are fixed to a mounting plate 102a on the base frame 102 so that each injection unit is aligned with one of the sprues 109 of the mold 105.
The prior art apparatus shown in FIG. 2 is provided with tilted injection cylinders 111 having nozzles 110. Nozzles 110 are aligned with sprue apertures 109a of sprues 109 located in the front of mold 105. Sprues 109 are in flow communication with cavities H of mold 105. By tilting injection cylinder 111, the distances between cavities H provided in the mold 105 can be made to be as narrow as possible and, thus, a small mold can be utilized (see the specification of the Japanese Patent application No. 58-194017).
The above two prior art apparatus have shortened the resin passage by omitting the mold runner without using a gate releasing device such as a hot runner etc. in the mold. However, the sprues are fixed, and the sprue-to-sprue distance is constant and cannot be changed.
Recently, some parts that had been difficult to injection-mold due to their shape, thickness, material, etc., have been made of resin instead of metal. In such injection molding, much emphasis is put on control by injection pressure and rate to optimize dimensional accuracy and mechanical strength. A major factor in a mold design that is intended to eliminate weaknesses at weld lines is the location of the gates.
Determination of the positions of the gates is based on the experience of the mold designer and the know-how accumulated in the mold technique field: the qates and sprues must be set at these proper positions without these positions being affected by the requirements of the injection molding apparatus.
These circumstances require a universal multi-cylinder type injection molding apparatus that has the capability to be used with various molds having different sprue positions.
One object of the present invention is to provide a multi-cylinder type injection molding apparatus that is capable of adjusting the nozzle positions of a plurality of injection cylinders in response to the positions of a plurality of sprues provided in a mold.
Another object of the invention is to provide an apparatus that decreases the defect rate of moldings by permitting the injection molding conditions in the injection units to be set independent of one another.
A further object of the invention is to provide an apparatus that has a shorter pitch between injection cylinder nozzles and can also be applied to a mold with a small sprue-to-sprue interval.
Additional objects and advantages of the present invention will be set forth in part in the description that follows and in part will be obvious from that description or can be learned by practice of the invention. The objects and advantages of the invention can be realized and obtained by the apparatus particularly pointed out in the appended claims.